Quadrature amplitude modulation (QAM) is an intermediate frequency (IF) modulation scheme in which a QAM signal is produced by amplitude modulating two baseband signals, generated independently of each other, with two quadrature carriers, respectively, and adding the resulting signals. The QAM modulation is used to modulate a digital information into a convenient frequency band. This may be to match the spectral band occupied by a signal to the passband of a transmission line, to allow frequency division multiplexing of signals, or to enable signals to be radiated by smaller antennas. QAM has been adopted by the Digital Video Broadcasting (DVB) and Digital Audio Visual Council (DAVIC) and the Multimedia Cable Network System (MCNS) standardization bodies for the transmission of digital TV signals over Coaxial, Hybrid Fiber Coaxial (HFC), and Microwave Multi-port Distribution Wireless Systems (MMDS) TV networks.
The QAM modulation scheme exists with a variable number of levels (4, 16, 32, 64, 128, 256, 512, 1024) which provide 2, 4, 5, 6, 7, 8, 9, and 10 Mbit/s/MHz. This offers up to about 42 Mbit/s (QAM-256) over an American 6 MHz CATV channel, and 56 Mbit/s over an 8 MHz European CATV channel. This represents the equivalent of 10 PAL or SECAM TV channels transmitted over the equivalent bandwidth of a single analog TV program, and approximately 2 to 3 High Definition Television (HDTV) programs. Audio and video streams are digitally encoded and mapped into MPEG2 transport stream packets, consisting of 188 bytes.
The bit stream is decomposed into n bits packets. Each packet is mapped into a QAM symbol represented by two components I and Q, (e.g., n=4 bits are mapped into one 16-QAM symbol, n=8 bits are mapped into one 256-QAM symbol). The I and Q components are filtered and modulated using a sine and a cosine wave (carrier) leading to a unique Radio Frequency (RF) spectrum. The I and Q components are usually represented as a constellation which represents the possible discrete values taken over in-phase and quadrature coordinates. The transmitted signal s(t) is given by: EQU s(t)=I cos (2.pi.f.sub.0 t)-Q sin (2.pi.f.sub.0 t),
where f.sub.0 is the center frequency of the RF signal. I and Q components are usually filtered waveforms using raised cosine filtering at the transmitter and the receiver. Thus, the resulting RF spectrum is centered around f.sub.0 and has a bandwidth of R(1+.alpha.), where R is the symbol transmission rate and a is the roll-off factor of the raised cosine filter. The symbol transmission rate is 1/n.sup.th of the transmission bit rate, since n bits are mapped to one QAM symbol per time unit 1/R.
In order to recover the baseband signals from the modulated carrier, a demodulator is used at the receiving end of the transmission line. The receiver must control the gain of the input amplifier that receives the signal, recover the symbol frequency of the signal, and recover the carrier frequency of the RF signal. After these main functions, a point is received in the I/Q constellation which is the sum of the transmitted QAM symbol and noise that was added over the transmission. The receiver then carries out a threshold decision based on lines situated at half the distance between QAM symbols in order to decide on the most probable sent QAM symbol. From this symbol, the bits are unmapped using the same mapping as in the modulator. Usually, the bits then go through a forward error decoder which corrects possible erroneous decisions on the actual transmitted QAM symbol. The forward error decoder usually contains a de-interleaver whose role is to spread out errors that could have happened in bursts and would have otherwise have been more difficult to correct.
Generally, in transmitting a modulated signal, the received signal at the demodulator has been amplified by a suitable amplification factor in order to compensate for attenuation in the received signal due to a transmission path or other factors. It is therefore necessary to control the amplification of the signal to control the received level of the signal. In order to control the level of the signal, often an automatic gain control (AGC) circuit, which controls the gain of the amplifier supplying the demodulator, is employed. For example, U.S. Pat. No. 5,729,173 to Sato discloses a QAM demodulator for receiving a QAM signal having a suppressed pilot signal. (Pilots exist in VSB modulations, but in QAM modulation, no pilot is necessary, and in general they are not used.) The demodulator includes an amplification factor controller in which the control of the amplification factor is performed separately from the control of the sampling timing for a received signal. Also, U.S. Pat. No. 5,761,251 to Wender discloses a circuit arrangement for achieving both DC offset correction and automatic gain control for QAM modulation. In many of the prior art designs, the automatic gain control (AGC) circuit is based uniquely on the QAM signal but with feedback to other analog circuits. This can cause the problem of saturation of the analog-to-digital converter circuit used at the input of the demodulator. Alternatively, other prior art designs use an AGC circuit that is based uniquely on the full input signal. However, this requires that the input signal has to have been perfectly filtered from adjacent channels prior to being input to the demodulator.
It is the object of the present invention to provide a QAM demodulator that provides gain control both prior to the demodulator, in order to prevent signal distortion caused by amplifier non-linearity and A/D saturation, and within the demodulator, to adapt the level of the QAM signal to the correct level with digital gain.
It is a further object of the invention to provide a QAM demodulator wherein the gain control is independent with respect to adjacent channels, and is therefore independent with respect to the symbol rate of the input signal.
It is a further object of the invention to provide a QAM demodulator with gain control that does not saturate the A/D converter.